1. Field of the Invention
This invention relates to electrochemical cells and is concerned more particularly with current collectors for such cells.
Many types of electrochemical cells contain materials which chemically are highly reactive or corrosive. In cells which employ a solid conducting membrane consituting the electrolyte and separating anodic and cathodic materials which are fluids, current collectors have to be provided to effect electronic current connection with the anodic and cathodic reagents. Corrosion of such current collectors is a problem if either or both the anodic and cathodic materials are highly reactive.
The present invention is concerned more particularly with sodium-sulphur cells. In such a cell, molten sodium is separated by a conductive membrane, usually beta-alumina, from a molten sulphur/sodium polysulphides mixture constituting the cathodic reactant. The sulphur/polysulphides is not only very corrosive, but it also has low electronic conductivity. Few materials with high electronic conductivity are stable in this cathodic environment. The sulphur/polysulphides material has a low electronic conductivity and, to improve this, it is the common practice to pack carbon or graphite felt in the cathodic region between the surface of the electrolyte and the surface of a cathode current collector. The distance between the current collector and the electrolyte surface is made as short as possible consistent with providing sufficient volume of cathodic reactant. For this reason, it is convenient to make the cell tubular with the electrolyte in the form of a tube having the cathodic reagent inside the tube and the current collector comprising a rod located in and extending along the tube. The graphite felt, although not having a high conductivity, does provide adequate conductivity over the short distance between the electrolyte material and the current collector. The current collector rod, which must be in multiple contact with the felt along the length of the cathodic region, has to provide a low resistance path to an output terminal. This rod however is exposed to the cathodic reactant and must be chemically inert with respect to the sulphur/polysulphides.
2. Prior Art
It has been proposed to use, as such a current collector, an impermeable graphite tube containing metal to give adequate longitudinal conductivity along the length of the rod. With such arrangements, however, one of the problems is to make good contact between the graphite tube and the metal core. This can be achieved by a deformable interface between the graphite tube and the core, as described for example, in United States Specification No. 3,982,957. This leads to a complex construction.
In cells where the sodium is inside the electrolyte tube and the cathodic reactant outside the electrolyte (known as central sodium cells) the problem is even more serious since the outer housing is used as the cathode current collector; any failure of this housing leads to leakage of the cathodic reactant. The direct application of a protective coating to a substrate of good conductivity might appear to be a solution to this problem but, although a number of such arrangements have been proposed, they have all had serious drawbacks. It has been proposed for example to use a stainless steel housing with a protective coating of molybdenum or carbon. It is difficult however to obtain satisfactory protective coatings of these materials. In such a construction, the integrity of the cell is dependent on the coating giving complete protective for the stainless steel. Many other ideas have been proposed. For example, U.S. Pat. No. 3,413,150 proposes the use of aluminum with a coating of chromium or titanium or chromel (a nickel-chromium alloy), such coatings being applied by electroplating or the decomposition of metal salts or vacuum deposition. Considerable difficulties have been met in attempting to put such ideas into practice, and it is still the common practice to use carbon or graphite in spite of its poor electrical conductivity. It is now well-known for example that titanium will rapidly corrode when used in the cathode electrode of a sodium-sulphur cell. Chromium and nickel-chromium alloys have not been found satisfactory as a cathode current collector because of problems with electronic exchange between the electronically conductive member and the electrode, resulting in only mediocre utilisation of polysulphide material. A chromium sulphide Cr.sub.2 S.sub.3 layer forms on chromium or chromium alloys in the cathodic reactant. Prior to cell operation however, a corrosion-resistant film of chromium oxide Cr.sub.2 O.sub.3 tends to form on chromium or chromium alloys; this is an electronic insulator and so leads to an increase in cell impedance.